Adding a way to disable all proxy processing (#691)

[GulajavaMinistudio]

• Adding a way to disable all proxy processing

When the proxy field in configuration is === false all proxy processing is
disabled. This specifically disable the 'http_proxy' environment variable
handling.

Fixes axios#635
Related to axios#434

• Change readme wording

From review comment on PR (axios#691)

#### Instructions

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• Create an issue explaining the feature. It could save you some effort in case we don't consider it should be included in axios.
• If you're fixing a bug, try to commit the failing test/s and the code fixing it in different commits.
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^^^ Delete the instructions before submitting the pull request ^^^

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